The present invention relates to plastic metal working and can be used in the machine building industry for the manufacture of workpieces from sheets, sections, and monolithic and welded panels forming a working surface of single or double curvature. A method is well known in prior art to be used for forming a workpiece under conditions when its material creeps (see, for instance, U.S. Pat. No. 3,739,617). A blank is placed on a heated die and pressed thereto over the entire surface thereof by means of a diaphragm. Then the die is heated uniformly so that it reaches a predetermined temperature. The blank is loaded by blowing air into the diaphragm (i.e., by differential pressure) so as to pressurize the diaphragm continuously over the entire surface of the blank until it fits completely the die.
However, when such forming is effected in accordance with this method of prior art knowledge by applying a uniform force (caused by the pressure built up in the diaphragm), a number of various deformations cannot be realized as necessary for producing the workpieces having complicated configurations. A continuous uniform force applied to the blank fails to ensure high precision of the finished workpiece when it is made from a blank having different rigidities within various portions thereof. Because of the uniform continuous heating of the die, some portions of the blank, if it has variable thickness and rigidity, can get heated up unevenly--a factor which is detrimental to the accuracy of the finished workpiece and which increases the additional stresses. For these reasons, it is impossible to obtain such strength characteristics of the workpiece material that are high enough, since the stresses emerging in the processes of straining may be higher than the limit of elasticity for this material so that plastic fractures may result which lead to a reduction in the strength properties of the workpiece material.
Also, another method is well known in prior art to be used in accordance with Inventor's Certificate Specification Serial No. 1147471, Int. Cl. B21D 11/20, wherein a blank is fixed in a plurality of points by means of movable rods arranged to be disposed coaxially with each other, then heated up to a predetermined temperature and deformed by moving the rods. This ensures the deformation of metal around the contour defined by the end faces of the stationary rods arranged to be disposed on the side of the workpiece bottom surface.
However, when such forming is effected in accordance with this method of prior art knowledge, the force applied to the fixed points of the workpiece throughout the entire process of deformation does not allow to realize a number of various deformations as necessary for producing the workpieces having complicated configurations. The deviation from the predetermined configuration seems to increase also due to the fact that it is actually impossible to make an exact allowance for the springing action since there are differences both in the geometrical parameters and in the thermal physical properties between various portions of the blank, i.e., the optimum conditions of deformation are not observed within some portions thereof--a factor which contributes to a reduction in the precision of forming as well as in the quality of the workpiece and its strength properties.
The method as taught by Inventor's Certificate Specification Serial No. 1147471 is essentially the nearest one to the method now claimed as far as the material features thereof and the useful results attainable are concerned so that it is, therefore, this particular method that has been selected by us to be the most representative one of the state of prior art.